The present invention relates generally to leakage current compensation devices for integrated circuit current sources and, more specifically, for a compensation device operating in a higher than normal temperature range.
It is well-known that the operating characteristics of semiconductor devices vary with heat. The leakage current of elements rises approximately exponentially with temperature. Thus, it is well-known to provide bypass devices for digital circuits and compensation elements for analog circuits to handle the large leakage current. Earlier attempts have been made to provide bypass through resistive elements. This has been found undesirable and the use of semiconductor devices for the bypass element has been suggested.
A typical example of the semiconductor element being used as a bypass element to compensate monolithic integrated current sources is described in U.S. Pat. No. 4,028,564 to Streit et al. Of particular interest are the embodiments of FIGS. 5a and 5b. In FIG. 5a, a transistor 29 with a short circuited base emitter diode is used effectively as the same as the base collector diode of FIG. 4e. To sink much greater currents, FIG. 5b shows a transistor 30 with an open base connection such that amplification factor is provided for the leakage current such that the geometry of transistor 30 may be smaller than that of transistor 1 whose base current is to be sunk.
The temperature environment for which Streit et al were designing was normal operating temperatures of the integrated circuit. These types of circuits have generally been temperature tested for the military range which is -55.degree. C. to +125.degree. C.
There is a growing need for electronics which operate in the range of 125.degree. C. to 300.degree. C. Such applications include well logging, jet engine control and industrial process controls. These temperatures are substantially higher than the normal operating range and even the military range. Prior art circuits have not addressed the problems associated with these extreme temperature environments. For example, PN junction leakage currents double approximately every 11.degree. C. and can reach microamp levels above 200.degree. C. even for small junctions. Currents of this magnitude may defeat the operation of sensitive low level circuitry.
The resistivities of the various regions of the integrated circuit increase with temperature. The lightly doped regions such as IC substrates and collector regions of transistors have the largest temperature coefficient, increasing approximately a factor of four over the 25.degree. C. to 200.degree. C. range. Transistors designed to remain in the linear region of operation will often saturate when operated above their design temperature at minimum collector-base voltage due to this effect.
The forward voltage of a PN diode has a strong negative temperature coefficient of about 2 mv/.degree.C. At 200.degree. C. V.sub.F of a diode and V.sub.BE of a transistor drop below 250 mv. Devices whose bias is generated from diode voltages may saturate due to this effect. Furthermore, the noise margin of the many logic circuits which depend upon them drop to very low values at temperatures above 200.degree. C.
H.sub.FE increases with temperature. A typical integrated circuit NPN H.sub.FE will somewhat more than double as temperature is increased from 25.degree. C. to 200.degree. C. A device with a high H.sub.FE operating at a low collector current such as might be found, for example, in the front end of an operational amplifier may experience a reversal of base current due to collector base leakage under these conditions.